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tedkidd said 2 moisture barriers = bad idea

I agree if we are talking about 2 layers of wrap But
in this case with the bubble wrap next to the duct OR next to the wrap's FSK depending on which approach they take, what I see is a really extensive vapor barrier NOT a sandwich with alternate layers of absorbent insulation with non absorbent layers of vapor barrier.

shophound, didn't know the term nucleation til now. I knew it had to be called something!
La rater, what he said sounds good. Its absolute moisture content that makes it "DRY" Relatively speaking of course

You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

ScorpionLeather

I believe that some moisture(vapor not liquid) will begin to collect in the space between the duct and FSK of just one layer of wrap. This will become a problem when too much vapor collects and finds a dew point temp that will turn this vapor top liquid. With the OFF cycles, the moisture/ vapor has a chance to dissipate. If there are 2 vapor barriers then this moisture would tend to be captured and become a problem unless you were to cut and rip the FSK of the wrap layer you are trying to add to.

Do you follow my logic? See any flaws in my thinking?

You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

I've seen ductwork with double vapor barriers
the insulation condensates between the vapor barriers
and the insulation value is zero.

I wouldn't ever do it. even on hard pipe the ductwrap should
be single layer..lots of folks insulate over existing ductwrap
as an 'upgrade' this actually causes more problems as the
hard pipe has lots of leakage sites and this moisture is drawn
into the duct system.

not a good idea...and in my hot humid climate...a really really
bad idea.

imo it is a lot of labor for no benefit.
your mastic sealing will provide the air seal.

best of luck

Yes... I heard long long ago that if you have two vapor barriers that moisture will collect between them. When double wrapping something you need to slice the vapor barrier of the first layer to allow air to ciruclate between the barriers.

I wouldnt use bubble wrap for any insulating. You cant use it in our area.

Eli hasn't posted since his second post.
hopefully we haven't run him off.

there was lots of information in both his posts that seem to have
gotten lost in the pages that followed.
reading his posts would be of value to addressing his specific questions
& concerns.
below are the questions from his two posts on page one of this thread.
but reading all of both posts will give info that is not included below.

Eli's first question:
If I wrap the ducts with the foil bubble wrap insulation (marketed as radiant barrier) directly on the ducts
while sealing it with aluminum tape and then wrap over that the aluminum backed fiber glass insulation,
aside from this possibly just being unnecessary and obsessive, is there any detrimental affects that could
occur from condensation (rust/mold) within the layers if applied as mentioned?

Eli's second question from his second and last post:
The manufacturers of the bubble foil even recommend using the bubble foil as the material for the spacers to
get the air gap in many cases... so, if condensation that would cause rust would be common from this,
I probably would've heard about it or read about it in another thread somewhere?

Eli's third question:
So... it is somewhat comforting that you say there is an expectation of the fiber glass's vapor barrier
being imperfect... with this in mind, if the bubble foil outer layer were to receive condensation at the
fiberglass contact, it probably would dry out quick enough or be as if it were in direct contact with the
duct metal making it no more of a condensation hazard than it would be without the bubble foil in place?
__________________________________________________ ________________________
1st question:
it is unnecessary. I'm not touching the obsessive part.
it will cause condensation. over time the duct will rust but it takes years
and will not be visible from the attic as it will be covered.
best option is mastic seal all seams joints & 90's. the insulate with
R-8 ductwrap.

perhaps not everyone has unwrapped old metal ducts.
sometimes the duct wrap just falls off..these are on the ducts that have been
taped..some of that old duct tapes works surpisingly well and adheres for years
as it is not directly exposed to attic temps.
other times there is no attempt to seal any of the duct leakage when the system
was installed. in these cases the insulation has wet and dried over the years and the
fiberglass sticks to the metal duct. you literally have to scrape it off to mastic seal.

question 2:
mfg websites are selling products. you won't see information that belittles their products.
cutting the fbf into 2" sections and wrapping the duct twice every 2' doesn't give you
the air space needed. this is 1/4" at best not the 3/4" they state it will be. hype.
don't buy into it. use that fbf as you radaint barrier. (even though fsk works just as well
with foil facing into attic space and attached to undersides of roof rafters..and costs less)

there has been information in threads/posts on this site that have touched upon this topic.
usually that information is buried deep within the pages of these discussions.
most don't read threads in their entirity before posting, have not come across this situation,
live in a different climate, or many other reasons.

question 3:
the duct wrap you are removing, as you stated in your posts is in bad shape.
the foil scrim kraft paper backing of your ductwrap is missing pieces and is torn in other areas.
this allows the moisture to exit that has been caused by duct leakage.

as the unit stops running the ducts suck in attic air through unsealed joints & seams.
this causes dirt trails on the insulation that correspond with duct leakage sites on ducts.
this hot air comming into contact with the insulation then sucked into the unsealed areas
causes condensation.
as you have mastic sealed these leaks, this condition is eliminated.
but there should be no exposed metal, so the ductwrap should be in full contact with the
plenum or these areas will condensate.

correctly installing new duct wrap with fsk intact will be the only vapor barrier you need.
all seams after being staped with a duct wrap stapler should be covered with fsk tape
to complete the vapor barrier

fyi, foil bubble wrap is a plastic material.
fsk foil scrim kraft is a paper. this is what you see on the back of duct wrap.
different materials have different properties & applications.

best of luck

The cure of the part should not be attempted without the cure of the whole. ~Plato

still here

i was surprised to see so much discussion on here tonight... i had started putting the ducts back together last weekend and hadn't seen any comments or received any more email notifications, so i assumed it fizzled out.

really interesting information, vapor and barriers and hot and cold and all the imperfections and possibilities still have me in a bit of a stupor, but there is a little bit more clarity on this specific application now... since it's a heat pump that'll be running in the wintertime, i don't even want to try to think about the impact of this setup in the reverse season. :-)

it is a retrofit at my own home i'm doing and since i hadn't heard back i did end up wrapping some with the bubble foil, but still haven't gotten to the fiberglass or even completing a main part of the plenum with the foil. i was afraid of the "will create rust over years" and the "it's against code" comments. i'm just trying to think of how forgiving some of these materials will be over time with the imperfections (i'm trying as hard as i can to get perfect seals all the way through, but even with just the fiberglass i see there being tooons of imperfections with all the twist and turns and attachments at the plenum and boots. i used a can of foam at each boot and then did go forward with the bubble wrap on the branches and am going to be living in the house for about 2 more years so, i'll get to monitor the different sections and even deconstruct one eventually. what i know right now is that what i'm doing is about 1000x better than what was there. the old fiberglass backing has tearing all over it and there were cracks everywhere in the ducts and the old fiberglass was very dirty because of this.

i'm thinking that if can get a radiant barrier up and bring the attic temperature down significantly and continue to try to get that perfect seal and i might be able to avoid mold n rust depending on the level of error forgiveness my climate and specific materials will allow. i did ask the original questions thinking about the long term as many of you seemed to have picked up on. i'm not really worried about the next 2 years, i'm thinking if this could actually go 20 years and be better off then the horrendous state of what i saw as i began to deconstruct for the retrofitting. my guess is that by the time any problems from what i'm doing present themselves the next owner will be a little wealthier than i am and that the materials available locally and the science of it all will be more understood by the town folk n contractors. and also just to clarify further now that there's been an open discussion: the whole reasoning behind using the bubble foil does even have anything to do with the radiant blocking or insulation properties... it has more to do with creating an additional layer of sealing around the duct to avoid any imperfections in my mastic sealing and aluminum taping of the metal from creating infiltration points that pull fiber glass and asbestos dust or just attice air into the ducts and the home. i'm realizing quickly that there are imperfections throughout it as the labor is just so frickin intense and hard to get right in some spots.

so... yeah... i may have just made some serious mistakes on some sections but scorp and the guy talking about the difference between what he actually sees in the field vs the theory gave me a little reassurance that'll go along with the fact of knowing that i am doing the job much better than it had been before. i'll monitor areas over time and who knows... if i end up settling in this house i'll search out this thread 10 years from now when i'm hiring some kid to redo all my rusting, molding ductwork and post an update. :-) i really do appreciate the insight and the discussion. VAPOR is something that has been fogging my brain for a long time... particularly with all these home improvement projects i'm doing and building science research i've been doing on the side for mine and my future family's benefit. i just want some air that's cold damnit! i'm tired of being without a central unit (this'll be my 3rd summer if i don't get it fired up soon) ... :-)

again... thanks. i hope ya'll were able to learn something (nucleation???) as well. take care... i love the internet because of forums like this. also, i don't want anyone to think the DIYer like myself that drops in here are trying to steal professional advice or trade secrets... i'm just trying to clarify information with people who might have a little more hands on field experience than me. if there's anyone in dallas / fort worth that's been a part of this thread send me a private message and i can give you a little work with my system. i don't want to do any of the silver soldering myself (don't own the torches) and could use help perfecting the air flow settings and refrigerant levels, checking for leaks, etc. i've done all the textbook stuff and a majority of the field work on my own, but there is always a place in the DIYer's life for the pro's to step in and take over.

20 years? Do you honestly have a 20 year perspective? Rare for an American homeowner.

HVAC equipment that is out doors (outside your thermal envelope) is absurdly wasteful and problematic. Long term the energy and maintenance, not to mention potentially expensive moisture issues, are HUGE.

Spray 3" closed cell foam to your roof deck ("Redefining" the space your equipment is in to "In Doors") and go have a beer.

Double wrap

Eli
Sorry I missed this thread at it's birth.

But I like the way you think. I just do it the opposite of how you did.I line the interior of the duct with insulation and then wrap it with bubble wrap.This stuff is great at deflecting radiant heat. I have a duct system in an un-insulated attic that consistently gets to outdoor ambient PLUS ++++.
There is only 1 degree on gain in 60' of duct run....

How did it turn out?

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